The present invention relates to an apparatus for effecting a focus control for use in an optical disc player, and more particularly to an improvement in an apparatus for detecting focus information with the aid of a reflection surface which is set at or nearly at a critical angle with respect to an incident light beam reflected by an optical disc. Such an apparatus has been disclosed in a copending U.S. patent application Ser. No. 195,075 now U.S. Pat. No. 4,390,781 filed on Oct. 8, 1980.
In the focus detecting apparatus of a kind described in the preamble, a light beam emitted from a light source is made incident on an optical disc as a light spot, the light beam reflected by or transmitted through the optical disc is made incident upon a reflection surface which is set substantially at a critical angle with respect to an optical axis and the light beam reflected by the reflection surface is further introduced onto a photodetector having divided light receiving regions. Then, focus information representing a deviation of the light spot in the direction of the optical axis with respect to the optical disc is derived by detecting whether the reflected light beam is collimated, diverged or converged. In the in-focused condition, the reflected light beam becomes a parallel beam, whereas when the light spot deviates from the disc surface, the reflected light beam becomes diverged or converged depending upon a direction of the deviation. In this known apparatus, since a variation in amount of the reflected light beam in accordance with the focusing condition is detected photoelectrically, a detection accuracy is not always optimum.
FIG. 1 is a block diagram showing a principal construction of the known focus detecting apparatus for effecting a focus control. In FIG. 1, a laser light 1' emitted from a laser light source 1 is collimated by a collimator lens 2 into a parallel beam and is then focused onto an information track having a crenellated pit construction recorded on a disc 5 as a light spot by an objective lens 4 through a half mirror 3. The optical disc 5 is rotated by a motor M at a given constant speed such as 1,800 rpm. A light beam reflected by the disc 5 is collected by the objective lens 4 and is made incident upon a prism 8 through the half mirror 3. The light beam is further reflected by a reflection surface 8A of the prism 8 which surface is set substantially at a critical angle with respect to an incident light beam. Then, a light beam reflected by the prism 8 is made incident upon a photodetector 7. In FIG. 1, there is also shown a plan view of the photodetector 7 having four light receiving regions 100-103 divided in two orthogonal directions X and Y, X being in parallel with a track direction. In the in-focused condition, the light beam reflected by the reflection surface 8A is made incident upon a center of the photodetector 7, so that each of four regions receives the same amount of light. If the light spot projected onto the disc 5 is deviated from the in-focused condition, position and cross sectional shape as well as intensity distribution of the light beam impinging upon the four regions 100-103 are varied, and a light amount received by each of the regions 100-103 is varied accordingly. Photoelectrically converted output signals of four light receiving regions 100, 101 and 102, 103 are supplied to analog adders 9 and 10, respectively. Output signals of the adders 9 and 10 are supplied to a differential amplifier 11 so as to obtain a difference output signal representing focus information. The difference output signal thus obtained is amplified by a current amplifier 11 to derive a focusing control signal which is then supplied to a moving coil 6. Then the moving coil 6 moves in the optical axis direction in accordance with the detected focus information. The moving coil 6 is fixed to the objective lens 4, and therefore a distance from the disc surface to the objective lens 4 can be controlled in accordance with a magnitude of the current passing through the moving coil 6. As a result, a negative feedback loop for effecting the focus control is constructed to maintain the in-focused condition by controlling a position of the objective lens 4 in a most suitable position.
However, in the known apparatus mentioned above, photoelectrically converted output signals of the four light receiving regions 100-103 of the photodetector 7 do not represent the pit information but an average value of the light intensity projected onto the regions 100-103. Although there is a certain correlation between the light amount and a reproduced level of the pit information, i.e. the reproduced RF signal, they are not completely identical with each other. Moreover, in the in-focused condition, since the light spot having the smallest diameter is projected onto the track recorded on the disc and the optical resolution is highest, a variation of the light amount modulated by the pit on the disc, i.e. the reproduced RF signal level becomes maximum and thus, it is not always possible to detect the in-focused position accurately only by detecting the variation of the light amount. Further, the apparatus is so constructed that in the in-focused condition, the amounts of the light projected onto the light receiving regions 100, 101 and 102, 103 illustrated in FIG. 1 become identical with each other. Thus, the focus information is subjected to a variation or fluctuation due to an unevenness of a reflection factor of the disc surface and a variation of a detection sensitivity due to an inclination of the disc surface. Owing to the various reasons explained above, the known apparatus has a drawback in that the focusing control cannot be performed accurately and precisely.